1. Field of the Invention
The invention relates generally to the field of marine seismic survey systems. More specifically, the invention relates to structures for sensor groups in marine seismic streamers having particle motion sensors and pressure sensors.
2. Background Art
Marine seismic survey systems known in the art include sensor cables called “streamers” towed in a body of water to acquire seismic signals resulting from actuation at selected times of one or more seismic energy sources in the water. Streamers include a plurality of seismic sensors disposed at spaced apart locations along the streamer. Typically, the sensors are arranged in a plurality of spaced apart groups or “arrays”, wherein in each such group or array a selected number of sensors are disposed at relatively close spacing (e.g., 0.7 meters), and the signals from all the sensors in the group are combined in some manner to generate the equivalent of a single sensor signal. The sensor groups may be separated from each other by a relatively longer spacing (e.g., 12.5 meters) to provide the equivalent of signals from a plurality of individual sensors disposed at spaced apart locations along the streamer. Sensor grouping and signal combination as explained above is used to reduce certain types of noise in the signals detected by the seismic sensors.
One type of seismic streamer known in the art called a “dual sensor” streamer includes both particle motion responsive sensors (e.g., geophones) and pressure or pressure time gradient responsive sensors (e.g., hydrophones) disposed in substantially collocated groups. One example of such type of streamer is disclosed in U.S. Pat. No. 7,239,577 issued to Tenghamn et al. and commonly owned with the present invention. The particular uses for such dual sensor streamers are well described in the foregoing patent.
It is known in the art that in seismic streamers having particle motion sensors, including dual sensor streamers, the particle motion sensors are susceptible to noise generated by the movement of the streamers in the water. One technique for processing dual sensor seismic streamer signals that takes account of such noise is described in U.S. Pat. No. 7,359,283 issued to Vaage et al. and commonly owned with the present invention. The method described in the foregoing patent includes synthesizing low frequency (e.g., below about 20 Hz) particle motion signals using signals from the pressure responsive sensors, and adding the synthesized low frequency components to measurements made by the particle motion sensors above the synthesized signal frequency limit. Particle motion signals measured by the particle motion sensors below the synthesis frequency limit are effectively not used in such method. As a result, potentially valuable low frequency particle motion information may be unobtainable.
One way to reduce noise in the lower part of the seismic frequency band in signals detected by the motion sensors is to use single sensor measurements. To do this it is necessary to have relatively densely spaced sensors that preferably are equally spaced over the group length. Such sensor arrangements have not been practicable due to limitations of power consumption along the streamer and limitations in handling large volumes of sensor data.
There continues to be a need for improved structures and signal processing techniques for seismic streamers including particle motion sensors that can attenuate the effects of motion induced noise in the particle motion sensor signals.